Microwave firing of MnZn-ferrites

Materials Science and Engineering B 106 (2004) 289 294 Microwave firing of MnZn-ferrites V. Tsakaloudi, E. Papazoglou, V.T. Zaspalis Laboratory of Inorganic Materials, Chemical Process Engineering Research Institute, Center for Research and Technology-Hellas, P.O. Box 217, 57001 Thessaloniki, Greece Received 30 June 2003; accepted 30 September 2003 Abstract Microwave firing is evaluated in comparison to conventional firing for MnZn-ferrites. For otherwise identical conditions, microwave firing results to higher densities and coarser microstructures. Initial magnetic permeability values (25 khz, 25 C, <0.1 mt) after conventional firing are approximately 5000, but the corresponding values after microwave firing are approximately 6000. Unlike the conventional firing process, the final density after microwave firing is increased by increasing the prefiring temperature. As appears from the results of this study, microwave firing could be in principle a promising MnZn ferrite firing technology for materials to be used in high magnetic permeability applications. No advantages of microwave firing are evident for materials intended to be used in high field power applications. 2003 Elsevier B.V. All rights reserved. Keywords: Ceramics; Ferrites; Microwave firing 1. Introduction Microwave firing of ceramics is a technology that has attracted much scientific interest as a technology for ceramic manufacturing [1,2]. Important advantages of microwave firing in relation to conventional firing are reported to be the higher densities [3,4], the lower firing temperatures [5,6] or the ability to manufacture nanocrystalline microstructures with interesting mechanical, electrical or other properties [7 10]. Microwave firing could be an interesting firing technology for polycrystalline magnetic ceramics, such as MnZn-ferrites, for several reasons. Higher densities are desired for all magnetic properties such as saturation magnetisation, magnetic permeability or electromagnetic power losses. Microwave firing can be considered as a direct heating method and is therefore expected to be more economic. The energy consumption should be reduced because the ferrite material is heated directly and less energy is wasted to heat the kiln. Corresponding author. Tel.: +302-310-498-114; fax: +302-310-498-131. E-mail address: zaspalis@cperi.certh.gr (V.T. Zaspalis). Microwave firing allows the achievement of high heating rates that cannot be achieved by conventional furnaces; this may lead to significant reduction of the firing cycles times. In order to evaluate in as quantitative a sense as possible the extent of the validity of the previous considerations, polycrystalline MnZn-ferrites have been synthesized both by conventional and by microwave firing techniques. The results of this study are reported in the following paragraphs. 2. Experimental MnZn-ferrites of the cubic spinel structure having the general formula (Mn 0.5 Zn 0.5 )Fe 2 O 4 are made as follows. Mn 3 O 4, ZnO and Fe 2 O 3 raw materials (Merck, Analytical Grade) are weighed in the amounts required by the chemical formula and are subsequently mechanically mixed for 5 10 min in the presence of about 5 wt.% humidity. Each synthesis batch is calculated to a total weight of 500 g mixed powder. After drying, the mixture is then prefired under air to 900, 1200 and 1380 C for 3 h. The prefired powders are ball milled in 50 50 solid water wt.% aqueous suspensions and with 10 12 mm stainless-steel ball diameter, for 9 13 h. The average particle size after milling, measured by laser scattering was 0.8 1.0 m. In each batch, prior to 0921-5107/$ see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.09.043

290 V. Tsakaloudi et al. / Materials Science and Engineering B 106 (2004) 289 294 milling, the same amount of 0.05 wt.% CaO is added as a dopant in the form of high purity CaCO 3 (Aldrich, Analytical grade). The milled powders are subsequently granulated in a homemade roll-granulator with the addition of 10 wt.% of a binder consisting of a 2 wt.% aqueous solution of polyvinyl alcohol (Merck, Analytical grade, molecular weight 72,000). Ring-shaped specimens with an outside diameter of ca. 25 mm, an inside diameter of ca. 15 mm and a height of ca. 12 mm are formed by pressing the granulated powder at compaction pressures of 500 kg/cm 2 using a specially designed hydraulic press. The density of the specimens after compaction is estimated geometrically to be 2.6 g/cm 3. The density of the specimens after firing is estimated by the Archimedes method using distilled water as the immersion fluid. Conventional firing of the specimens is made in specially constructed computerized kilns allowing for independent programming of the temperature and oxygen partial pressure profiles. This is in order to achieve equilibrium partial pressure of oxygen during cooling that as known is required during firing of MnZn-ferrites [11]. The specimens are placed on ZrO 2 refractory tiles next to each other. Microwave firing of the specimens is made in a microwave furnace equipped with a 2.45 GHz generator and an adjustable power supply. Since the microwave furnace was not capable of performing cooling under equilibrium oxygen partial pressure conditions, from the top temperature until the end of the firing cycle the atmosphere is changed from air to nitrogen. This is to preserve the spinel structure from oxidation and irreversible formation of the hematite phase, that is favoured if cooling takes place under high partial pressures of oxygen. For the magnetic property measurements, the Fe 2+ /Fe 3+ ratio for the microwave samples is then subsequently adjusted by re-heating up to 900 C followed by cooling under stoichiometric conditions in a conventional kiln. Since the difference between the top firing temperature (1400 C) and the re-heating temperature (900 C) is quite large it is assumed that no changes in the morphology and density of the specimens occurs upon reheating, that might influence significantly the magnetic results. However, the morphological characterization of the microwave sintered specimens is done on non re-heated specimens. The specimens for the microwave firing are placed, next to each other on top of Y 2 O 3 stabilized ZrO 2 refractory tiles that are in turn placed on top of SiC plates. It seems that SiC nicely absorbs the microwave energy during the heating cycle and thus may also provide indirect heating to the products placed on it. However, SiC cannot be in direct contact with ferrites at temperatures exceeding 1300 C because of undesirable chemical interactions between the refractory and the ferrite. The total loading of the microwave furnace is kept constant during the experiments. The temperature and atmosphere profiles used for the various firings are shown in Fig. 1. The temperature and atmosphere profiles used for the conventional firing correspond to curves (a) and (a ), respectively. The temperature and atmosphere profiles used for the microwave firing are shown by curves (a) and (a ), respectively. Since microwave firing allows the achievement of very high heating rates, an experiment has been performed also using the full power of the microwave generator and leading to the profiles corresponding to curves (b) and (b ). Microstructural characterization of the sintered specimens is done with the Scanning Electron Microscope (SEM) Fig. 1. Temperature (solid lines) and oxygen partial pressure (dashed lines) profiles used during firing. Conventional firing corresponds to curves (a) and (a ); microwave firing corresponds to curves (a) and (a ); microwave firing at high heating rates corresponds to curves (b) and (b ).

V. Tsakaloudi et al. / Materials Science and Engineering B 106 (2004) 289 294 291 on epoxy imbedded cross-sections of the specimens subjected to chemical etching for grain boundary visualization. Chemical analysis around the grain boundary region is done with a Transmission Electron Microscope equipped with Energy Dispersive Analysis of X-rays (TEM-EDAX). Average grain sizes (based on statistically cut two-dimensional cross-sections) and grain size distributions are evaluated by using special software (analysis) capable to process digitized microscope images. For the evaluation of the real grain-sizes from randomly cut two-dimensional cross-section images, the values obtained should be multiplied by a statistically derived constant factor. However, this is omitted since it does not have an influence upon any of the relative and comparative conclusions in the following paragraphs. Specimens for electromagnetic property evaluation are wound with copper wires in order to form inductor coils with 10 12 turns. The electromagnetic properties are mea- sured from the phase shift and current of the input and output signal, with a phase impedance analyzer, equipped with power amplifiers, frequency generator and oscilloscope. 3. Results and discussion In Table 1, the final densities are shown for the fired specimens that are prefired according to the standard procedure at 900 C. As can be concluded from Table 1, microwave firing produces higher final densities than the conventional firing process. Moreover, the higher the heating rate of the microwave firing process, the higher the final density of the samples. In Fig. 2, typical SEM microstructure images are shown. The accompanying cumulative grain size distributions as obtained by digital processing of the images are shown in Fig. 3. Defining as d x the equivalent spherical grain size Table 1 Final specimen densities, grain sizes, initial magnetic permeabilities and saturation magnetizations as a function of the firing process used Sintering process Conventional (4.3 C/min, as shown in Fig. 1, curves (a) and (a )) Microwave (4.3 C/min, as shown in Fig. 1, curves (a) and (a )) Microwave (7.5 C/min, as shown in Fig. 1, curves (b) and (b )) Final density a (g cm 3 ) d 50 ( m)/d 90 ( m) Initial permeability b (25 C, <10 khz, <0.1 mt) 4.79 7.5/12.5 5000 380 4.84 10/15.5 6000 400 4.93 10.5/18.5 5500 330 a Density values are the average of five specimens with absolute error of 0.01 g cm 3. b Shown values are the average of five specimens with relative error of 2%. Saturation magnetization (mt) b (25 C, 250 A m 1, <10 khz) Fig. 2. Typical digitised microscopy images of cross-sections of specimens subjected to (a) conventional firing with a heating rate ca. 4.3 C/min; (b) microwave firing with a heating rate of ca. 4.3 C/min; (c) microwave firing with a heating rate of ca. 7.5 C/min.

292 V. Tsakaloudi et al. / Materials Science and Engineering B 106 (2004) 289 294 Fig. 3. Cumulative equivalent spherical grain diameter distribution for specimens fired: (a) conventionally with a heating rate of ca. 4.3 C/min, as shown in Fig. 1, curves (a) and (a ); (b) in a microwave oven with a heating rate of ca. 4.3 C/min, as shown in Fig. 1, curves (a) and (a ); (c) in a microwave oven with a heating rate of ca. 7.5 C/min as shown in Fig. 1, curves (b) and (b ). (i.e. diameter) below of which lies x% of the total number of grains on the two-dimensional surfaces of the pictures, then the results shown in Table 1 can be calculated. As is indicated by the results, microwave firing favours the development of coarser polycrystalline microstructures when compared to conventional firing. The higher the heating rate of the microwave firing process, the coarser the microstructure or the larger the average grain size. The final densities as a function of the prefiring temperature for comparative conventional and microwave firing experiments are shown in Fig. 4. As can be seen, although higher prefiring temperatures do not favour the achievement of high final densities in conventional firing, the reverse is true for microwave firing of MnZn-ferrites. In Table 1 the initial magnetic permeability (at a field intensity of <0.1 mt) and the saturation magnetization values are shown, measured at a temperature of 25 C and a frequency of 10 khz. A typical high-resolution trans- mission electron microscopy picture of a grain boundary is shown in Fig. 5. Chemical analysis measurements (EDAX) on lines crossing the grain boundaries, as is indicated in Fig. 5, on the Ca-dopant and Si-impurity concentration are shown in Fig. 6 for the specimens fired in a microwave oven. At otherwise identical conditions, microwave firing of MnZn-ferrites results in higher densities and coarser microstructures (Table 1, Figs. 2 and 3). This is an observation quite similar to those made earlier on other types of polycrystalline ceramic materials [3 6]. The coarser character of the microstructure obtained by microwave firing increases as the heating rate increases (Table 1, Figs. 2 and 3). This is of importance and should be taken into account in the case that microwave firing is considered as the faster alternative to conventional firing. Another interesting observation is that the coarser the microstructure the higher the concentration of isolated pores inside the grains. This can be taken as an Fig. 4. Final densities as a function of the prefiring temperature for specimens fired: (a) in a microwave oven with a heating rate of ca. 4.3 C/min, as shown in Fig. 1, curves (a) and (a ); (b) conventionally with a heating rate of ca. 4.3 C/min, as shown in Fig. 1, curves (a) and (a ).

V. Tsakaloudi et al. / Materials Science and Engineering B 106 (2004) 289 294 293 Fig. 5. Typical high resolution transmission electron microscopy photo showing a grain boundary having a width of ca. 10 nm and the direction along which chemical analysis line scans are taken. indication of higher grain boundary mobility during firing under microwave conditions. The dependency of the final densities on the prefiring temperature (Fig. 4) shows a quite remarkable behaviour. Usually, for the same compaction densities and firing schedules, the final density decreases as the prefiring temperature decreases. The reason is the loss of powder reactivity which occurs when prefiring takes place at elevated temperatures and which prevents the compacted powder from reaching high densities during the final firing stage. This behaviour is Fig. 6. Typical Ca (solid lines) and Si (dashed lines) concentration profiles across a grain boundary for specimens fired: (a and c) in a microwave oven with a heating rate of ca. 4.3 C/min, and (b and d) in a microwave oven with a heating rate of ca. 7.5 C/min, as shown in Fig. 1. followed by the conventionally fired specimens as indicated by curve (b) in Fig. 4. The situation is reversed when firing is done in a microwave oven. A possible explanation can be the fact that the higher prefired material (which contains higher concentrations of the magnetic spinel phase) couples better with the electromagnetic energy of the microwave generator. This result indicates that the relations between the process operation parameters change and consequently, the process optimisation is subjected to different rules, when the firing of the material is done under microwave conditions. Another consequence is that microwave firing seems to allow the usage of high prefiring temperatures without serious densification problems. An advantage of using high prefiring degrees is the achievement of high compact green densities (because of the high solid densities) and consequently the execution of firing under low shrinkage conditions which is in favour of better product dimensional control. Also, the higher the prefiring temperature the more solid state chemical reactions towards the formation of the final phase occur at this process step, therefore the less reactive the final firing step. It is well known from laboratory experience that the homogeneity and the smoothness of the microstructure development is favoured when densification and grain growth during sintering are not accompanied by side chemical reactions. The latter lead quite often to microstructure deterioration due to pore growth. The difference in the magnetic properties between identically fired specimens under conventional and microwave conditions (Table 1) are in agreement with the expectations based on their morphological properties [12]. The higher density and the coarser microstructure achieved by microwave firing favour both higher saturation magnetisation values and higher magnetic permeability values. An exception to the previous statement can be seen in Table 1 on the properties of the fast microwave fired specimens. Despite their higher densities and average grain diameters both the initial permeability and saturation magnetisation values are lower than the corresponding slower heated specimens. It is believed that the answer lies to the distribution of dopants, impurities and other cations in the microstructure. As shown in Fig. 6 the accumulation of both Ca and Si (that constitute the basic foreign cations) at the grain boundaries, takes place to a greater extend in slowly heated specimens. In specimens fired under fast microwave conditions a significant amount of those foreign cations is still present in the lattice. It can therefore be assumed that when those non-magnetic cations occupy regular or even interstitial positions, disturbing thus the periodicity of the cubic magnetic matrix, a number of important properties such as crystal magnetic anisotropy, are being negatively influenced, resulting to worsening of the magnetic properties. The accumulation of foreign cations at the grain boundaries occurs through a diffusional process involving solution in the matrix at the elevated firing temperatures and gradual precipitation at the grain boundaries upon cooling [13]. Most probably, under fast microwave firing conditions there

294 V. Tsakaloudi et al. / Materials Science and Engineering B 106 (2004) 289 294 is not enough time provided for the previous process to occur [14]. In this way, the differences in the concentration profiles shown in Fig. 6 can be understood. MnZn-ferrites for power applications under alternating currents require a highly resistive phase at the grain boundaries in order to reduce the generation of ohmic eddy currents. They constitute an electromagnetic energy dissipation factor and reduce the efficiency of the components. This phase is usually created through the addition of certain doping elements to the material that during firing will preferentially accumulate homogeneously along the grain boundaries [15]. As appears from the results of this study, fast microwave firing is most probably not a suitable fast alternative for the manufacturing of such magnetic components, despite the higher density advantage. Moreover, the coarsening of the microstructure is not always desirable for power applications since it may reduce the electromagnetic resonance frequency and therefore enhance the resonance losses [16]. Applications requiring high induction levels require high permeability magnetic materials. They are usually associated with low magnetisation fields so that electromagnetic power losses do not play a dominant role. Best performance is usually achieved with pure raw materials and with polycrystalline microstructures consisting of high densities, large grains and as pure as possible grain boundaries. Foreign ion migration processes are in fact absent during processing of these types of materials. Based on the results reported in this study it is expected that microwave firing could provide significant advantages either in terms of high densities and large grains or in terms of a faster firing process. 4. Conclusions Microwave firing is compared to conventional firing for the manufacturing of MnZn-ferrites. Microwave firing results in higher densities and coarser microstructures when compared to conventional firing. The density and the coarse character of the microstructure in microwave firing increase with increasing heating rates. However under fast firing conditions the time provided to the foreign cations for accumulation at the grain boundaries might not be enough. Although advantages of microwave firing for the manufacturing of MnZn ferrite materials for power applications are not evident, the technology shows promising aspects for the manufacturing of high permeability materials. References [1] W.H. Sutton, Microwave processing of ceramic materials, J. Am. Ceram. Soc. Bull. 68 (1989) 376. [2] D.E. Clark, W.H. Sutton, Microwave processing of materials, Annu. Rev. Mater. Sci. 26 (1996) 299. [3] N.A. Travitzky, A. Goldstein, O. Avsian, A. Singurindi, Mater. Sci. Eng. A2& 6(2000) 225. [4] Y. Fang, Y. Chen, M.R. Silsbee, D.M. Roy, Mater. Lett. 27 (1996) 155. [5] A. Goldstein, N. Travitzky, A. Singurindi, M. Kravchik, J. Eur. Ceram. Soc. 19 (1999) 2067. [6] C. Siligardi, C. Leonelli, F. Bondioli, A. Corradi, G.C. Pellakani, J. Eur. Ceram. Soc. 20 (2000) 177. [7] Z. Xie, C. Wang, X. Fan, Y. Huang, Mater. Lett. 38 (1999) 190. [8] Z. Xie, J. Yang, X. Huang, Y. Huang, J. Eur. Ceram. Soc. 19 (1999) 381. [9] S.T. Oh, K. Tajima, M. Ando, Mater. Lett. 48 (2001) 215. [10] I. Lin, W. Lee, K. Liu, H. Cheng, M. Wu, J. Eur. Ceram. Soc. 21 (2001) 2085. [11] R. Morineau, M. Paulus, IEEE Trans. Magn. 5 (1975) 1312. [12] A. Goldman, Handbook of Modern Ferromagnetic Materials, Kluwer Academic Publishers, Dordrecht, 1999. [13] S.H. Chen, S.C. Chang, C.Y. Tsay, K.S. Liu, I.N. Lin, J. Eur. Ceram. Soc. 21 (2001) 1931. [14] V. Zaspalis, R. Mauczok, M. Kolenbrander, J. Boerekamp, in: P. Vincenzini (Ed.), Proceedings of the 9th Cimtec-World Ceramics Congress, Part E, 1999, p. 401. [15] V. Zaspalis, E. Antoniadis, E. Papazoglou, V. Tsakaloudi, L. Nalbantian, C. Sikalides, J. Magn. Magn. Mater. 250 (2002) 98. [16] E.C. Snelling, Soft-Ferrites; Properties and Applications, Butterworth & Company, London, 1988.